1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image reading apparatus and, more particular, to positional control of a sheet stacking member which can be lifted or lowered.
2. Description of the Related Art
Conventionally, an image reading apparatus such as a scanner, an image forming apparatus such as a printer or a facsimile, or a composite machine having the functions thereof in combination has been provided with a sheet feeding apparatus which feeds a sheet such as an original or a recording sheet to an image reading portion or an image forming portion. For example, a conventional image reading apparatus includes a liftable sheet stacking member, on which a sheet-like original is placed, original feeding unit, and a sheet feeding apparatus which feeds an original stacked on the sheet stacking member to an image reading portion by the original feeding unit (See Patent Document 1: Japanese Patent No. 3747986; and Patent Document 2: Japanese Patent No. 3560803).
The conventional sheet feeding apparatus includes a detecting unit which detects whether or not a sheet stacked on the sheet stacking member reaches a sheet feed position, so as to control lifting of the sheet stacking member based on a result detected by the detecting unit. This control enables the sheet feed position to be constantly maintained irrespective of the number of sheets set on the sheet stacking member, thereby stabilizing a sheet feeding operation.
By way of an example of the positional control of the sheet stacking member, if it is detected that an uppermost sheet stacked on the sheet stacking member has not reached the sheet feed position, the sheet stacking member is lifted by a predetermined amount. Here, the detection of the position of the sheet feeding unit which is contacting to the uppermost sheet allows detection as to whether or not the sheet stacked on the sheet stacking member reaches the sheet feed position.
By way of another example, if it is detected that the uppermost sheet does not reach the sheet feed position, the sheet feeding operation is temporarily brought to a halt, and then, the sheet stacking member is lifted until it is detected that the sheet stacked on the sheet stacking member reaches the sheet feed position.
However, in the sheet feeding apparatus, the image reading apparatus, and the image forming apparatus in the prior art, the position of the sheet feeding unit may be fluctuated by an adverse influence of vibrations of the apparatus, a curl, a fold, a crease of the sheet, or a deformation of a roller provided with the feeding unit, or the like. When the position of the sheet feeding unit is fluctuated, a sensor which detects the position of the sheet feeding unit erroneously detects the position of the sheet feeding unit.
In this manner, if the position of the sheet feeding unit is erroneously detected, the sheet stacking member may be lifted farther by a predetermined amount from the erroneously detected position in the case where the position of the sheet stacking member is controlled by detecting the position of the sheet feeding unit. As a consequence, the sheet may be brought into press-contact with the sheet feeding unit, thereby possibly making it difficult to feed the sheet.
In order to solve the above-described problem, it has been proposed that there is provided, for example, an upper limit detecting sensor or an abutment which restricts the movement of the sheet feeding unit by a torque limiter, such that the sheet feeding unit cannot be moved upward beyond the correct sheet feed position. However, if such an upper limit detecting sensor or the like is provided, the apparatus may be increased in size and cost.
In the meantime, in the case where the position of the sheet stacking member is controlled based on the position of the uppermost sheet stacked on the sheet stacking member, the sensor may erroneously detect due to the vibrations of the apparatus that the uppermost sheet does not reach the sheet feed position irrespective of the reach of the uppermost sheet up to the sheet feed position.
Even in the case of such an erroneous detection, the sheet stacking member is lifted. At this time, the feeding operation must be temporarily brought to a halt during lifting of the sheet stacking member. In other words, if the sensor erroneously detects the position, the sheet stacking member may be unnecessarily lifted with a temporary halt of the feeding operation, thereby reducing productivity.
In order to solve the above-described problems, there has been proposed a configuration in which a sensor for detecting that the uppermost sheet reaches the sheet feed position is located at a position apart from the sheet feeding portion and irrelevant to the sheet feeding operation, so as to prevent any halt of the sheet feeding operation.
However, in this case, the sensor may not accurately detect the position of the upper surface of the sheet by the influence of the status of the sheet such as an end float caused by the curl, so that the sheet may not be properly fed. Here, there is provided sheet pressing unit in such a manner as to prevent any influence of the status of the sheet such as the end float caused by the curl, thereby inducing an increase in size and cost of the apparatus.
The present invention has been made in view of such circumstances and provides a sheet feeding apparatus which can stably feed a sheet and prevent any degradation of productivity, an image reading apparatus, and an image forming apparatus.